* If `toArray` is inherited from Java, it may take an argument or
return a value of a flexible type, which looks nullable in IR;
* The returned array may also have `out` variance of the type
argument;
* `Array<T>` is not the same as `Array<Any?>` if `T` is neither `Any`
nor `in Something`, so presence of `toArray(): Array<T>` does not
mean we don't need to generate a new `toArray`.
This allows us to not generate redundant immutable collection
stubs. The code to generate the immutable collection stubs does
not deal well with thinking that all external declarations
come from Java.
